A monitoring/control/data-acquisition system should be expandable according to the number of points needed, so as to require just a few modules or many modules. In addition, a system design that allows modules to be concentrated into a single location (such as a rack) or be distributed near the point of use reduces wiring costs. Failures in one module should not affect the operation of other modules and modules should be able to be swapped out without affecting other modules. High speed communication between modules is required. In this regard, serial communication keeps pin counts low and bandwidths high, but high speed serial links (such as Ethernet) are typically point to point and require a bridge or switch circuit. This switch circuit becomes a single point failure source that can take the entire system out of operation
In existing monitoring/control system designs, the single point failure issue has been solved using redundant networks. In the case of standard PICMG 2.16, two separate switch cards (referred to as “fabric cards”) are installed into the rack holding monitoring/control cards, providing separate Ethernet links to each card. However, there are difficulties to with this approach. First, a switch card must have enough ports on it to service the full rack. For a small number of cards, the user must still purchase the two switch cards, thereby greatly increasing the buy-in cost of the rack. Second, the size of the rack is limited to the number of channels supported on the switch cards.
Other designs have solved the problem using a loop architecture in which the end module of the chain loops back to the first. This architecture requires identification of the last module in the chain and a long wiring run from the last module to the first.